fix: system health is a Settings tab, not a page only the dot reached

The surface shipped at /system with no nav entry, reachable only by
clicking the health dot beside the brand — a target you have to already
suspect something is wrong to go looking for. Operator-flagged: it needs
a path someone can walk to.

Settings is where you go to ask the instance about itself, so the view
becomes a tab there, beside Activity — Activity answers "what is the
queue doing", System answers "is anything left to do it".

- SystemView.vue moves to components/settings/SystemHealthTab.vue; the
  content is unchanged apart from shedding its own container and h1.
- SettingsView adopts useTabQuery (the composable Browse and
  Subscriptions already use) so a tab can be linked TO. The health dot
  now points at ?tab=system, and /system redirects there so the previous
  build's link and any bookmark still land.
- The tab drops its own 10s poll. v-window keeps a visited item mounted
  rather than destroyed, so that timer would have gone on firing behind
  Maintenance — and TopNav already polls the same store every 15s for
  the dot.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TTjbZZ6JirCMSaJzQV1RhA

frontend/src/router.js

@@ -1,6 +1,5 @@
 import { createRouter, createWebHistory, createMemoryHistory } from 'vue-router'
 import SettingsView from './views/SettingsView.vue'
-import SystemView from './views/SystemView.vue'
 import GalleryView from './views/GalleryView.vue'
 import ShowcaseView from './views/ShowcaseView.vue'
 import ExploreView from './views/ExploreView.vue'
@@ -46,12 +45,13 @@ const routes = [
 
   // Settings — config, pinned to the right of the nav (TopNav special-cases it).
   { path: '/settings', name: 'settings', component: SettingsView, meta: { title: 'Settings', stickyChrome: true } },
-  // Deliberately NO meta.title: TopNav builds its nav row from routes that
-  // have one, and this is reached from the health indicator beside the
-  // brand — the place someone already looks when they suspect something is
-  // wrong. A sixth top-level tab for a page you visit twice a year would
-  // cost more attention than it returns.
-  { path: '/system', name: 'system', component: SystemView },
+  // System health is a Settings TAB, not a route of its own (operator
+  // 2026-09-02). It first shipped as /system reachable only from the health
+  // dot, which is a target you have to already suspect something to go
+  // looking for. Settings is where someone goes to ask the instance about
+  // itself. The path stays as a redirect so the dot's old link, and any
+  // bookmark from that build, still land somewhere real.
+  { path: '/system', name: 'system', redirect: () => ({ name: 'settings', query: { tab: 'system' } }) },
 
   // The old standalone paths now redirect into the Browse hub, preserving any
   // deep-link query (e.g. /posts?post_id=N → /browse?tab=posts&post_id=N). The